Power devices having a high breakdown voltage and a large current capability are used in various fields. A power device has a problem in that the temperature of the power device increases due to power losses, which causes changes in its device characteristics. In order to reduce such changes in the device characteristics, a structure has conventionally been adopted which cools the power device so as to be retained at or below a safe operating temperature. Specifically, the power device is kept in contact with a package base-material, and the heat generated in the power device is allowed to pass into the package base-material, thus minimizing increase in the device temperature.
A power device utilizing a silicon semiconductor (Si power device) which has a band gap of about 1.11 eV at room temperature undergoes a thermal runaway at temperatures over 150° C. and thus is short-circuited, no longer functioning as a current controlling device. Therefore, an Si power device is subjected to a thermal design such that the temperature of a portion of the Si power device having the highest current density will never exceed 150° C. In particular, in the case where the current density inside the Si power device is 50 A/cm2 or above, a considerable heat is generated inside the Si power device, and thus it is necessary to efficiently dissipate the heat.
However, there exists a problem in that, even if the temperature of a power device is maintained at or below a safe operating temperature (150° C.), the electrical resistance of the power device while it is conducting (hereinafter referred to as an “ON resistance”) would change responsive to changes in temperature, whereby its reliability is degraded.
An Si-MOSFET (metal-oxide-semiconductor field-effect transistor) will be taken as an example of a conventional Si power device. In an Si-MOSFET, the package base-material itself and the method of mounting the Si-MOSFET onto the package base-material are optimized so that the Si-MOSFET will be retained at or below a safe operating temperature (150° C.) even when operated at full power. So long as the device temperature of the Si-MOSFET is kept at or below 150° C., device destruction will not occur.
However, when the device temperature increases to about 100° C., the ON resistance will usually have significant changes. FIG. 4 is an exemplary graph showing the temperature characteristics of an ON resistance RDS(on) of a conventional Si-MOSFET, which is disclosed in Non-Patent Document 1. As shown in FIG. 4, the ON resistance of the conventional Si-MOSFET increases as the temperature Tj of the device increases. The ON resistance value at 100° C. is twice or more of the ON resistance value at room temperature.
The reasons why the ON resistance of an Si-MOSFET increases with increases in temperature are that the ON resistance is mainly governed by the electrical resistance in a drift region of the Si-MOSFET, and that the electrical resistance of the drift region shows a large temperature dependence. The drift region is a region which contains impurities at a relatively low concentration. It is considered that, when the temperature increases, phonon scattering in the drift region increases to prohibit carrier conduction, thus resulting in an increased electrical resistance.
Temperature-induced changes in the electrical characteristics of an Si-MOSFET cause the following problems.
Circuits for controlling devices such as inverters are, generally speaking, power electronics circuits incorporating switching devices such as Si-MOSFETs. When the electrical characteristics of an Si-MOSFET changes with temperature, the current which flows through a load (e.g., an inverter) of the circuit also changes accordingly. Thus, when a current flowing through a load of a circuit exhibits temperature dependence, there is a problem in that the operation of a system which is controlled by that circuit becomes unstable. In order to allow the system to stably operate, it would be necessary to ensure that the same current will be supplied to the addition of the circuit even if the ON resistance of the Si-MOSFET increases as a result of a temperature increase of the Si-MOSFET, e.g., through a feedback control which increases the voltage. However, the circuit structure would be complicated and the manufacturing cost would increase with the provision of such a feedback control.
Si power devices other than Si-MOSFETs also permit their electrical characteristics to change with temperature, and have similar problems to the above. An Si-IGBT (insulated-gate bipolar transistor) is described as an example of another Si power device. The ON resistance of an Si-IGBT decreases with an increase in temperature. Therefore, when a circuit including an Si-IGBT is constructed, if the electrical resistance of the Si-IGBT decreases due to an increase in temperature of the Si-IGBT, it is necessary to perform a feedback control which lowers the voltage. Moreover, the ON resistance of an IGBT exhibits a greater temperature dependence than that of the ON resistance of an Si-MOSFET. Therefore, it is necessary to subject the Si-IGBT to a thermal design such that the heat generated due to a current flowing through the Si-IGBT during operation can be dissipated with an even higher efficiency, thus to maintain the temperature of the Si-IGBT at a low temperature.
As for SiC, characteristics evaluations of the ON resistance of MOSFETs are disclosed in Patent Documents 1 and 2, and Non-Patent Document 2.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-261275
[Patent Document 2] Japanese Laid-Open Patent Publication No. 7-131016
[Non-Patent Document 1] Infineon Technologies, Cool MOS Power Transistor data sheets SPP04N60C3, SPB04N60C3, SPA04N60C3
[Non-Patent Document 2] “Planar-type 4H-SiC MOSFETs having High Inversion Layer Channel Mobility”, FED Journal Vol. 11 No. 2 (2000), FIG. 3 on p. 82